Due to the remarkable advance in existing technologies for graphical reproduction that are commonly available on the market, the occurrence of counterfeits of good quality of security documents such as checks, banknotes, etc. are becoming common. Therefore, a certification and authentication method for security documents using methods that can withstand the technological advance of graphic equipment, and that makes use of commonly available devices such scanners or digital cameras, is of utmost importance.
Moreover, organized crime has found a way to use these professional technologies to produce large volumes of high-quality counterfeits; this fact jeopardizes the trust that society has in the issuing institution and the institution's ability to provide reliable instruments to detect counterfeits.
The commonly known authentication methods focused on verification based on devices or automatic equipment rely on processes and systems that require equipment with specific characteristics to verify the authenticity of one security feature of the document, where the difficulty is imposed either by a high cost of the equipment or the special material that is required. Examples are the inclusion of magnetic materials, UV fluorescent inks, infrared absorbing pigments, systems of dynamical optical response such as phosphorescence decay, magnetic bands and holograms which are included in security documents. Unfortunately, criminals are also using methods that are becoming more sophisticated, and with the help of modern ease of access to information and reproduction media, it is possible to imitate almost any security feature that manufacturers have incorporated into the documents by means of commercially available materials and equipment. The counterfeit of documents can include counterfeits based on systems using laser or inkjet printers (photocopy type counterfeits) and counterfeits using processes and materials similar to the genuine (counterfeits by process). In the case of photocopy type counterfeits, the central banks include special features that printers cannot imitate: for example, watermarks, magnetic threads, transparent windows, engraved reliefs with tactile properties, holograms and optically variable features and inks that change with the viewing angle. In the case of a counterfeits by process, the authenticity is difficult to determine by simple examination from a non-trained user. In addition, for the particular case of money counterfeiting, this has a high impact in the finances of a nation. Actually, the efforts to counteract such counterfeits are focused on the materials and inks to be used in the manufacture of new security documents, trying to foresee which of them will have a high risk of becoming commercially available during the intended lifetime of the document in order to avoid them. In general, the standard approach requires predicting when the gap between commercially available technologies and those intended for security documents is closing due to the inevitable technological progress.
In the case of currencies such as U. S. Dollars and Euros which are used in many countries, the risk of counterfeiting increases as there opens an opportunity to criminals of those countries, including state-sponsored criminal organizations.
Therefore, it is desirable that the authenticity of a security document does not only rely on the fact that the genuine materials and processes are difficult, however not impossible, to counterfeit; but also on mathematically proven security such as a digital certificate.
On the other hand, in the case of banknotes, which are extensively used for everyday transactions, it is necessary to be able to immediately determine their authenticity. As an example, if a person, during a transaction, fails to detect a counterfeit, this will result in a loss since he will receive a document with no value, deliver a valuable merchandise, and in some cases provide the change in cash when the value of the merchandise is lower than the value of the counterfeit. Thus, there is a requirement of being able to quickly authenticate security documents such as checks and banknotes to prevent the entrance of such forged documents into the national economy that affects the general public, and to keep trust in the issuing institutions.
Forensic tests are very accurate in determining whether a document is genuine or not; however, the processing time for one document is very large, and they usually require expensive and specialized equipment. On the other hand, computers and gadgets with constantly growing image acquisition quality and computing capacity are becoming more popular and more economically accessible to individuals and corporations. Thus, it would be desirable to authenticate security documents by the use of currently available technology for digital image acquisition in devices like smart phones or scanners connected to computers which can analyze data to check the authenticity of a security document, particularly in the case of banknotes.
It has also been recognized in the field of security documents testing that a balance must be struck between the conflicting goals of “acceptance” and “rejection”, positive acceptance being the ability to correctly authenticate—identify and accept—all genuine security documents no matter their condition, and positive rejection being the ability to correctly discriminate and reject all non-genuine security documents. However, there is a possibility of negative acceptance, wherein a non-genuine document is accepted as genuine; and negative rejection, wherein a genuine document is considered as a counterfeit and rejected. It is necessary for the authentication process to have a high level of accuracy regarding these definitions, thus avoiding negative acceptances and negative rejections.
U. S. Patent Application No. 2004/0268130 discloses a system and method of authenticating an article that includes the steps of selecting an inherent feature of the article, and converting the feature into digital data to form an identification code for the article. An encryption system is used to encrypt the identification code, utilizing a secret private key of an asymmetric encryption key pair, and associated with the issuing party. The encrypted code is made available on a label accompanying the article. During a subsequent phase and at an authentication station, digital data relating to the feature is determined directly from the article and the code is decrypted utilizing a public key of the pair obtained from a third party in accordance with rules of a public key infrastructure. According to the system and method of U. S. Patent Application No. 2004/0268130, the inherent feature is the result of a chemical manipulation of the article or the inclusion of unique characteristics in or on the article. It also requires that the encrypted code is available on a label accompanying the article; however, the code is encrypted without applying a hash function. This flaw will allow a counterfeiter to obtain the original data that defines the uniqueness of the article and to start making modifications in order to approach the original data. By using a hash function, the counterfeiter will have no information on how to modify the article to try to make it pass the authentication. Moreover, it is not clear whether the chosen characteristics will be able to withstand the natural deterioration from the use of the articles.
Advanced Track and Trace (ATT) has developed a method to certify printing plates, and their correspondingly printed banknotes. It uses robust cryptographic methods. However, it has a main drawback: the fact that the number of codes is restricted to the plates designed for the manufacture of banknotes using the ATT process; one can only reproduce the serials that were printed with the same plate, due to cryptographic protection. In addition, due to the complexity of the pattern that has to be printed, it requires a microscope or some augmenting device as well as an improved quality control, in order to minimize the variations of batches of security banknotes.
As can be noticed, there is a need for a method to certify security documents as well as a trustable and quick process for authenticating documents. In addition, with the always-increasing development in information technology, this method adds to the security and confidence in a physical security document with a digital certificate counterpart on the understanding that both are needed to assess the validity of the document. Particularly, in this scenario, considering that the physical document and the digital certificate are uniquely linked together, a banknote without a valid digital certificate will be of no value even though the document is genuine.